1. Technical Field
This invention relates to an antilock braking device for precluding the locking of the wheels of a vehicle on sudden application of the brake and a road surface friction sensor and a road surface friction coefficient detector which can be used as components of said antilock braking device.
2. Technical Background
The conventional antilock braking device for cars or other vehicles generally employs a system such that the braking action is automatically controlled according to the chassis speed and wheel speed in such a manner that the slip ratio will fall within a definite range (See, for example, Japanese Patent Publication No. 30585/1984 and Japanese laid-open Patent Application KOKAI No. 61354/1985). The relationship between road surface friction coefficient and slip ratio is variable according to the texture of the road surface and, for this reason, the above system does not always provide the maximum braking force depending on the condition of the road surface and, in such cases, does not insure the minimum braking distance. Furthermore, because the chassis speed value used is an approximate value estimated from the wheel speed, the precision of slip ratio control is not sufficiently high. In order to ascertain the exact chassis speed, one has to rely on complicated devices such as the ground relative speed sensor (for example, Japanese laid-open Patent Application No. 64861/1988) or chassis deceleration sensor (for example, Japanese laid-open Patent Application No. 170157/1988).
In the conventional antilock braking device described in Japanese laid-open Patent Application No. 25169/1988, the road surface friction torque acting on the wheel (tire torque) is calculated from the wheel angular acceleration and brake fluid pressure values and the beginning of a fall in tire torque during the elevation of brake fluid pressure is utilized as one of the criteria for ascertaining the condition immediately preceding a wheel lock. However, since the tire torque is indirectly calculated from the wheel angular acceleration and brake fluid pressure, the above system does not take care of indefinite constants such as the moment of inertia of the wheels, the braking efficiency of the brake and so on, thus presenting problems in terms of the accuracy of data. There also is the problem that since the distance between the wheel to the road surface varies according to the deceleration of the chassis depending on the pneumatic pressure of the tires and the weight of the chassis, the road surface friction force and the tire torque are not necessarily maintained in a fixed ratio.
It is an object of this invention to provide an antilock braking device free from the above-mentioned disadvantages of the conventional device.
It is another object to provide a road surface frictional force sensor and a road surface friction coefficient detector which can be used as components of said antilock braking device.
A first antilock braking device according to this invention includes a brake control means adapted to cyclically perform an operational series which comprises sensing the road surface frictional force, increasing the brake fluid pressure while the road surface frictional force is increasing in response to the elevation of brake fluid pressure, decreasing the brake fluid pressure when the road surface frictional force declines despite elevation of the brake fluid pressure, and increasing the brake fluid pressure again when the road surface frictional force decreases in response to a fall-off of brake fluid pressure. The road surface frictional force can be known from measured values of the tire strain or the strain around the wheel of the vehicle.
A second antilock braking device according to this invention includes a brake control means adapted to cyclically perform an operational series which comprises detecting the coefficient of road surface friction, increasing the brake fluid pressure while the road surface friction coefficient is increasing in response to the elevation of brake fluid pressure, relieving or releasing the brake fluid pressure as the velocity of gain in road surface friction coefficient falls below a set value and increasing the brake fluid pressure again after the road surface friction coefficient has declined below said set value. The road surface friction coefficient value used in this second antilock braking device can be calculated from the road surface frictional force value and the vertical load value obtainable from measured values of the tire strain or the strain around the wheel.
The relationship between wheel-road surface slip ratio and road surface friction coefficient can be represented by curves such as shown in FIG. 1. On the ordinary road surface, this relation can be expressed by a curve having a peak as shown at C1. On an extraordinary road surface, such as a snow-clad road surface the relation may be represented by a curve without a peak as shown at C2. Not only the presence or absence of a peak but also the height of the peak and the magnitude of the slip ratio corresponding to the peak vary with the condition of the road surface and the chassis speed. On the other hand, as represented by curve C3, the cornering force (lateral drag) decreases monotonously in response to an increase in slip ratio. Therefore, as far as trackless vehicles such as automobiles are concerned, in order to obtain the maximum braking force without sacrificing the cornering force, it is ideal to apply the brake in the neighborhood of P1 or P2 on curve C1 or C2 as the case may be.
Let it be supposed that the vehicle is running on a road surface such that the relation between road surface friction coefficient and slip ratio can be represented by the curve C1 shown in FIG. 1. It should be understood that the road surface friction force is approximately proportional to the road surface friction coefficient. Under these conditions, the first antilock braking device according to this invention functions as follows. First, as a sudden brake is applied by depressing the brake pedal or manipulating the brake lever, the brake fluid pressure increases. While the detected road surface frictional force value continues to rise, the brake fluid pressure is increased consistently to apply the brake with an increasing force. This phase corresponds to the segment to the left of P1 on the curve C1 shown in FIG. 1. As the brake fluid pressure is increased to apply the brake more forcefully, the slip ratio increases to approach to the point P1 of maximum road surface friction coefficient. As the brake fluid pressure is further increased, the point P1 is passed over in due course. Beyond P1, the locking of the wheels begins to occur as the road surface frictional force turns to decline against the elevation of brake fluid pressure. When the road surface friction sensor output decreases in this manner, the brake fluid pressure is decreased to releave the brake action. Therefore, the locking of the wheels is prevented. As the road surface frictional force decreases in response to a decline in brake fluid pressure, the brake fluid pressure is increased again. As the result of this action, as long as the vehicle runs on the road surface which can be represented by a curve with a peak in regard to the road surface friction coefficient-slip ratio relation, the locking of the wheels can be prevented irrespective of road condition and, moreover, the braking action making the most of road surface frictional force can be realized.
The frictional force which acts between each wheel of the vehicle and the road surface is dynamically equivalent to the braking force applied by the wheel on the chassis. Therefore, strains and stresses proportional to the road surface frictional force are generated in all given positions of the structure between the point of contact of the wheel with the road surface and the chassis. Therefore, it is possible for one to measure the structural strain at an appropriate point of the structure and detect the road surface frictional force through the strain value. The member of the structure in which the maximum strain is generated is the tire in case the vehicle has tires in its wheels. Therefore, the road surface frictional force can be detected from measured values of the tire strain. It is also possible to affix strain gauges to the bearing shaft supporting the wheel, for instance, and measure the strain around the wheel. This strain is smaller than the tire strain but since said shaft is not a rotary element, the construction of the road surface friction sensor can be simplified.
The vertical drag exerted by the road surface an each wheel, or the vertical load which the wheel applies to the road surface as a reaction thereto, can, for the same reason as above, also be detected from a measured value of the tire strain or the strain around the wheel.
The second antilock braking device according to this invention functions as follows. As the motorist depresses the brake pedal or manipulates the brake lever with a great force, the antilock braking device is started. In the segment to the left of P1 or P2 on curve C1 or C2, the road surface friction coefficient xcexc increases in response to an elevation of brake fluid pressure. However, when the velocity of gain in xcexc falls off below a predetermined reference level (slightly to the left of the point P1 or at the point P2), the brake fluid pressure is releaved or released, whereupon the value of xcexc turns to diminish. After a decline corresponding to a given proportion of the maximum value immediately preceding the beginning of decrease of the road surface friction coefficient xcexc, the brake fluid pressure begins to rise again. Thereafter, the above sequence of events is repeated. In this manner, not only when the vehicle is running on a road surface such that the relation between road surface friction coefficient and slip ratio traces the aforementioned curve C1 but also when the road surface can be represented by curve C2 without a peak, the road surface friction coefficient at application of the brake is maintained in the neighborhood of P1 and P2, thus insuring a more or less ideal braking action. For vehicles (rolling stock, etc,) which run on tracks, in which no cornering force is required, said predetermined reference value for the velocity of gain in xcexc is set at zero or an appropriate negative value. Then, the braking action utilizing the maximum road surface friction force can be insured. The road surface friction coefficient value to be used in this second antilock braking device can be found by computation from the above-mentioned road surface frictional force value and the vertical load value obtainable through tire strain data or the data of strain around the wheel.
Thus, according to the device of this invention, the braking distance can be minimized irrespective of the condition of the road surface and, at the same time, the object of an antilock braking effect can be accomplished. Furthermore, the device does not require a complicated setup for measuring the chassis speed.